Polyamines are biologically ubiquitous organic cations whose biochemical function is not precisely defined. The radical change in growth and in other properties of cells depleted of polyamines by genetic or pharmacological manipulation attests to their metabolic importance. In mammalian cells, ornithine decarboxylase (ODC) is the first metabolic step in the synthesis of polyamines. Its activity is usually rate limiting for that pathway. Stimuli that produce anabolic or mitogenic responses tend to increase activity, and these changes may exceed several hundred-fold. Multiple mechanisms probably underlie regulatory alterations of ODC activity. The low relative abundance of ODC in mammalian tissues has frustrated study of these phenomena. To facilitate such mechanistic investigations and to provide a high abundance source of ODC mRNA for purposes of gene cloning, we have selected mutant cells that overproduce the enzyme. This was done in S49 mouse lymphoma tissue culture cells by applying a multistep selection for resistance to difluoromethylornithine, an enzyme-activated suicide substrate. We thus obtained cells that, as a consequence of ODC gene amplification, produce more than 15% of their total protein as ODC. Using mRNA from these cells, we generated ODC cDNA clones. We have begun to use these to investigate the molecular biology of ODC regulation in a variety of mammalian systems. (B)